Another possibility is that the trend reveals a trade-off that varies with men's mating strategy, said Robert Martin, a biological anthropologist and author of "How We Do It: The Evolution and Future of Human Reproduction," (Basic Books, 2013).
"A man can either invest in looking after the child of one wife, or he can invest more in sperm production if he has several wives," Martin, who was not involved in the study, told LiveScience.
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